The present invention relates generally to semiconductor devices and, more particularly, to measuring the depth of a well in a semiconductor device or substrate.
The depth and shape of the boundary of a well affect the electrical properties of a power integrated circuit (IC), especially in Rds-on, the threshold voltage, and the drain-to-source breakdown voltage. Thus, monitoring the depth and shape of the boundary of a well is an important task.
Traditionally, the spreading resistance probe (SRP) is used to simulate the depth of a well. A well-skilled operator is needed, however, to prepare the sample prior to measurement with the probe. Moreover, the use of the probe for well measurement is time-consuming and expensive. If the width of the device region is smaller than 50 xcexcm, this method cannot be used.
In another technique, the secondary ion mass spectroscopy (SIMS) is used to analyze the distribution of dopants in the device. If the width of the device region is smaller than 100 xcexcm, however, such a method cannot be used.
The present invention is directed to an effective and relatively inexpensive way for measuring the depth of a well in a semiconductor device or substrate. In specific embodiments, the device having a well in a substrate is cut through the well, and different regions forming the well are selectively removed by an etchant to expose the boundary of the well. The depth of the well is measured by scanning electron microscopy (SEM) or other suitable techniques.
In accordance with an aspect of the present invention, a method for measuring the depth of a well of a substrate comprises providing a substrate having a well therein and a cut through a depth of the well. The substrate is exposed to an etchant to reveal a discontinuity in a boundary at the depth of the well. The depth of the well is measured at the boundary.
In some embodiments, the well is a p-type well and the substrate is an n-type substrate. In other embodiments, the well is an n-type well and the substrate is a p-type substrate. In some embodiments, the etchant may comprise nitric acid, hydrofluoric acid, saturated iodide, and deionized water. The iodide may be selected from the group consisting of potassium iodide (KI) and sodium iodide (NaI). In other embodiments, the etchant may comprise nitric acid, hydrofluoric acid, saturated iodine, and deionized water.
In specific embodiments, the substrate is dipped into the etchant to reveal the discontinuity in the boundary at the depth of the well. The etchant is at a temperature of about 20-30xc2x0 C. The substrate is dipped in the etchant for about 10-50 seconds. The substrate is washed with deionized water after exposing the substrate to the etchant to reveal the discontinuity in the boundary at the depth of the well. The depth of the well may be measured by a scanning electron microscope.
Another aspect of the present invention is directed to a method for measuring a depth of a boundary between a first conducting doped region and a second conducting doped region. The method comprises using an etchant to selectively etch the first conducting doped region and the second conducting doped region to reveal a boundary therebetween. The etchant has a different selectivity between the first conducting doped region and the second conducting doped region. The method further comprises measuring the depth of the boundary between the first conducting doped region and the second conducting doped region.
In some embodiments, the boundary is a p-n boundary. The etch rate of the second conducting doped region is higher than the etch rate of the first conducting doped region. The first conducting doped region may be a p-doped region and the second conducting doped region may be an n-doped region.
Another aspect of the present invention is directed to a method for measuring the depth of a boundary between a first n-doped region with a first doped concentration and a second n-doped region with a second doped concentration which is different from the first doped concentration. The method comprises using an etchant to selectively etch the first n-doped region with the first doped concentration and the second n-doped region with the second doped concentration which is different from the first doped concentration to reveal a discontinuity at a boundary therebetween. The method further comprises measuring a depth of the boundary between the first n-doped region and the second n-doped region.
In accordance with another aspect of the present invention, an etchant having an etch rate of an n-doped region which is higher than an etch rate of a p-doped region comprises the following:
1 molar ratio of nitric acid;
0.01-0.05 molar ratio of hydrofluoric acid;
2 molar ratio of deionized water; and
0.001-0.003 molar ratio of iodide ions.
In some embodiments, the etchant has an etch rate of a first n-doped region having a first n-dopant concentration which is higher than an etch rate of a second n-doped region having a second n-dopant concentration, wherein the second n-dopant concentration is lower than the first n-dopant concentration.
In accordance with another aspect of the present invention, a method for measuring depths of boundaries comprises using an etchant to selectively etch a p-doped region, a first n-doped region having a first n-dopant concentration, and a second n-doped region having a second n-dopant concentration which is higher than the first n-dopant concentration, to reveal a first boundary between the p-doped region and the first n-doped region, and to reveal a second boundary between the first n-doped region and the second n-doped region. The etch rates are characterized as follows:
an etch rate of the second n-doped region greater than 
an etch rate of the first n-doped region greater than 
an etch rate of the p-doped region.
The method further comprises measuring a depth of the first boundary and a depth of the second boundary. The depths may be measured using a scanning electron microscope.